U.S. patent application number 12/189914 was filed with the patent office on 2009-02-26 for oscillation device and inspection apparatus.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Masahiro Asada, Toshihiko Ouchi, Ryota Sekiguchi.
Application Number | 20090051452 12/189914 |
Document ID | / |
Family ID | 40381588 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090051452 |
Kind Code |
A1 |
Asada; Masahiro ; et
al. |
February 26, 2009 |
Oscillation device and inspection apparatus
Abstract
An oscillation device has a resonant tunneling diode formed by
interposing a gain medium including a first barrier layer, a
quantum well layer and a second barrier layer between a first
thickness adjusting layer and a second thickness adjusting layer.
The oscillation device also has a switch for switching the polarity
of a bias voltage being applied to the resonant tunneling diode.
The first thickness adjusting layer and the second thickness
adjusting layer have different thicknesses. Thus, a single
oscillation device is driven to oscillate with different
oscillation frequencies.
Inventors: |
Asada; Masahiro;
(Yokohama-shi, JP) ; Ouchi; Toshihiko;
(Sagamihara-shi, JP) ; Sekiguchi; Ryota;
(Kawasaki-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
TOKYO INSTITUTE OF TECHNOLOGY
Tokyo
JP
|
Family ID: |
40381588 |
Appl. No.: |
12/189914 |
Filed: |
August 12, 2008 |
Current U.S.
Class: |
331/107T |
Current CPC
Class: |
H03B 7/08 20130101 |
Class at
Publication: |
331/107.T |
International
Class: |
H03B 7/08 20060101
H03B007/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 2007 |
JP |
2007-213644 |
Claims
1. An oscillation device using a resonant tunneling diode
comprising: a resonant tunneling diode formed by interposing a gain
medium including a first barrier layer, a quantum well layer and a
second barrier layer between a first thickness adjusting layer and
a second thickness adjusting layer; and a switch for switching the
polarity of a bias voltage being applied to the resonant tunneling
diode, the first thickness adjusting layer and the second thickness
adjusting layer having different thicknesses.
2. The device according to claim 1, wherein the thicknesses of the
first and second thickness adjusting layers are not less than 5 nm
and not more than 60 nm.
3. The device according to claim 1, further comprising a first
electrode and a second electrode for applying the bias voltage to
the resonant tunneling diode, the switch being adapted to switch
the oscillation frequency of the oscillation device by inverting
the polarity of the bias voltage being applied to the resonant
tunneling diode relative to the first electrode and the second
electrode.
4. The device according to claim 1, wherein the gain medium
includes a plurality of quantum well layers and a further barrier
layer is interposed between the quantum well layers.
5. The device according to claim 1, wherein the first thickness
adjusting layer is a non-doped layer arranged between the first
barrier layer and the first electrode and the second thickness
adjusting layer is a non-doped layer arranged between the second
barrier layer and the second electrode.
6. The device according to claim 1, wherein the quantum well layer
is made of a material containing indium gallium arsenide and the
first and second barrier layers are made of a material containing
aluminum arsenide or indium aluminum arsenide, while the first and
second thickness adjusting layers are made of a material containing
non-doped indium gallium arsenide.
7. The device according to claim 1, further comprising an antenna
resonator.
8. An inspection apparatus for inspecting the presence or absence
of a substance to be detected by adjusting the oscillation
frequency of an oscillation device according to claim 1 to a
characteristic vibration spectrum of the substance to be
detected.
9. A communication system adapted to have communications by means
of a frequency-shift keying system, where an oscillation device
according to claim 1 is employed as light source and the
oscillation device is subjected to frequency switching.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a current injection type
oscillation device using a frequency region from the millimeter
wave to the terahertz wave (from 30 GHz to 30 THz). More
particularly, the present invention relates to a current injection
type oscillation device having a resonant tunneling diode
structure.
[0003] 2. Description of the Related Art
[0004] Non-destructive sensing techniques using electromagnetic
waves in a frequency region from the millimeter wave to the
terahertz wave (from 30 GHz to 30 THz) (to be referred to simply as
"terahertz wave" hereinafter) have been and are being developed. In
the field of application of electromagnetic waves of this frequency
band, safe imaging techniques have been and are being developed for
see-through inspection apparatus that can replace X-ray inspection
apparatus. Furthermore, spectroscopic technologies for looking into
the physical properties including the bonded status of a substance
through determination of the absorption spectrum and the complex
permittivity of the inside of the substance, biomolecule analyzing
techniques and techniques for evaluating the carrier concentration
and the mobility are on the way of development.
[0005] Additionally, inspection apparatus for inspecting the
presence or absence of a substance having an absorption spectrum
unique to the terahertz band, or so-called fingerprint spectrum,
are on the stage of preliminary development. Such an inspection
apparatus can operate quickly and efficiently by discretely
providing a plurality of oscillators having oscillation frequencies
that are close to the fingerprint spectrum of the object substance
of inspection (typically from 0.1 THz to 10 THz) because it does
not involve any sweep in the time domain or the frequency
domain.
[0006] THz wave generating unit include units for generating a
pulse wave by irradiating a femtosecond laser beam onto a
photoconductive device and parametric oscillation units for
generating a wave of a specific frequency by irradiating a
nanosecond laser beam onto a nonlinear crystal. However, such units
are all of the photoexcitation type that is accompanied by
limitations in terms of downsizing and reduction of power
consumption rate.
[0007] Structures produced by using a quantum cascade laser and a
resonant tunneling diode (RTD) are being studied as current
injection type devices that operate in the terahertz wave region.
Particularly, a resonant tunneling diode type device that are
disclosed in Japanese Patent Application Laid-Open No. 2007-124250
and Japanese Patent Application Laid-Open No. H08-116074 are
expected to operate at room temperature at or near 1 THz. Such a
device is typically formed by a quantum well of InGaAs/InAlAs that
is made to grow by epitaxial growth by means of a lattice-aligned
system on an InP substrate. It illustrates a negative resistance as
illustrated in FIG. 12 of the accompanying drawings for the
voltage-current (V-I) characteristic and the oscillation
characteristic and is observed to be oscillating near this
region.
[0008] A planar antenna structure formed on the surface of a
substrate as disclosed in Japanese Patent Application Laid-Open No.
2007-124250 is suitably employed as resonance structure for
oscillation. A cavity resonator having a three-dimensional
structure is formed in the case of the type disclosed in Japanese
Patent Application Laid-Open No. H08-116074. It is a structure
where the wall surfaces and the rear surface of the device are also
covered by an electrode.
SUMMARY OF THE INVENTION
[0009] Generally, the oscillation frequency cannot be changed to a
large extent in such semiconductor oscillation devices in the
terahertz wave region so that a large number of oscillation devices
having different oscillation frequencies need to be prepared in
order to apply such devices to inspections for identifying a
substance by means of a fingerprint spectrum in the inside of the
substance.
[0010] In the case of an antenna resonator type device of Japanese
Patent Application Laid-Open No. 2007-124250, a non-doped spacer
layer is arranged between the contact layers for establishing
contact with the electrodes at the opposite sides of the active
layer of the resonant tunneling structure and a depletion layer
where no carrier exists is formed in the spacer layer depending on
the thickness of the spacer layer. As a voltage is applied to the
device for an oscillation operation, the thickness of the depletion
layer formed in the spacer layer changes to in turn change the
oscillation frequency. However, the change in the oscillation
frequency is slight and about several % at most so that such a
device cannot accommodate a plurality of spectrums.
[0011] The device structure described in Japanese Patent
Application Laid-Open No. H08-116074 represents a three-dimensional
cavity resonator. Therefore, as well known, while the Q value of
the resonator is relatively large and the oscillation frequency
changes as the equivalent permittivity of the entire inside
changes, a single device cannot produce frequencies that differ to
a large extent.
[0012] An oscillation device using a resonant tunneling diode in
the first aspect of the present invention is characterized by the
following features.
[0013] It has a resonant tunneling diode formed by interposing a
gain medium including a first barrier layer, a quantum well layer
and a second barrier layer between a first thickness adjusting
layer and a second thickness adjusting layer and a switch for
switching the polarity of a bias voltage being applied to the
resonant tunneling diode. Additionally, the first thickness
adjusting layer and the second thickness adjusting layer have
different thicknesses.
[0014] An oscillation device in the second aspect of the present
invention is characterized by the following features. Namely, it is
an oscillation device provided with a gain medium having a gain of
an electromagnetic wave, a planar antenna type resonator for
confining an electromagnetic wave in the gain medium and a unit for
injecting carriers into the gain medium. The gain medium is formed
by one or more quantum well layers and a plurality of barrier
layers separating the quantum well layers and the transition of
carriers among sub-bands of the one or more quantum well layers is
based on the resonant tunneling diode where a gain is produced by
way of photon assisted tunneling. The unit for injecting carriers
includes first and second thickness adjusting layers sandwiched
between first and second contact layers made of a semiconductor
illustrating conductive properties and the barrier layers, the
first thickness adjusting layer and the second thickness adjusting
layer having different thicknesses.
[0015] An inspection apparatus in the third aspect of the present
invention is characterized by inspecting the presence or absence of
a substance to be detected by adjusting the oscillation frequency
of an oscillation device according to the present invention to a
characteristic vibration spectrum of the substance to be
detected.
[0016] A communication system in the fourth aspect of the present
invention is characterized by having communications by means of a
frequency-shift keying system, where the frequency of an
oscillation device according to the present invention is shifted by
way of a unit for switching the polarity of a bias voltage being
applied to a resonant tunneling diode in a manner as defined
above.
[0017] For the purpose of the present invention, the thickness
adjusting layer corresponds to a non-doped spacer layer of the
prior art. However, the thickness of a depletion layer where no
carrier exists is positively adjusted to control the
characteristics of the resonant tunneling structure device. By this
reason, it is referred to as a thickness adjusting layer. With such
an arrangement, the bias point where a negative resistance is
generated is changed to shift the oscillation frequency by
inverting the polarity of a voltage being applied to the
oscillation device relative to the first electrode and the second
electrode.
[0018] The oscillation wavelength can effectively be shifted when
the thickness adjusting layers are within a range not less than 5
nm and not more than 60 nm.
[0019] An oscillation device that is capable of oscillating with
one of two frequencies can be driven to change the oscillation
frequency with time by switching the polarity of the bias voltage
and selecting one of the two frequencies.
[0020] The present invention can provide an inspection apparatus
for inspecting the presence or absence of a substance to be
detected by adjusting the oscillation frequency of an oscillation
device according to the present invention to a characteristic
vibration spectrum of the substance to be detected.
[0021] The present invention can provide a compact and low power
consumption system including a sensor apparatus or an imaging
apparatus that operates in the terahertz region as described in the
Related Art section by using an oscillation device according to the
present invention.
[0022] Thus, a single oscillation device according to the present
invention can oscillate in two frequencies. When an inspection
apparatus for inspecting the presence or absence of a specific
substance by means of the fingerprint spectrum thereof is formed by
using oscillation devices according to the present invention, the
number of oscillation devices that are employed can be reduced if
compared ordinary arrangements where a considerable number of
oscillation devices that correspond to various spectrums are
required.
[0023] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIGS. 1A and 1B are a schematic perspective view of an
oscillation device according to the present invention and a
schematic cross sectional view of the semiconductor part thereof,
illustrating the structure thereof.
[0025] FIG. 2 is a graph illustrating the relationship between the
thickness of the collector layer and the oscillation frequency of
an oscillation device according to the present invention.
[0026] FIGS. 3A and 3B are a schematic illustration of the change
of state when the polarity of the bias voltage of an oscillation
device according to the present invention is switched.
[0027] FIG. 4 is a schematic equivalent circuit diagram of the
semiconductor part of an oscillation device according to the
present invention.
[0028] FIG. 5 is a graph illustrating the V-I characteristic of the
device of the first example.
[0029] FIG. 6 is a schematic perspective view of the oscillation
device of the second example.
[0030] FIG. 7 is a graph illustrating computation examples for
designing the oscillation frequencies of the oscillation device of
the third example.
[0031] FIG. 8 is a graph illustrating computation examples for
designing the oscillation frequencies of the oscillation device of
the fourth example.
[0032] FIG. 9 is a schematic illustration of the structure of the
active layer of the oscillation device of the fourth example.
[0033] FIG. 10 is a schematic illustration of the inspection
apparatus of the fifth example.
[0034] FIG. 11 is a graph schematically illustrating an exemplary
transmission spectrum of a substance to be detected.
[0035] FIG. 12 is a graph illustrating the characteristics of a
resonant tunneling diode type oscillation device.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0036] Now, an oscillation device employing a resonant tunneling
diode according to the present invention will be described below. A
resonant tunneling diode is employed for the purpose of generating
an electromagnetic wave in the terahertz band. The resonant
tunneling diode is formed by interposing a gain medium including a
first barrier layer, a quantum well layer and a second barrier
layer between a first thickness adjusting layer and a second
thickness adjusting layer as will be described in greater detail
hereinafter by referring to FIGS. 1A and 1B.
[0037] An oscillation device according to the present invention has
a switch as unit for switching the polarity of the bias voltage
being applied to the resonant tunneling diode. Additionally, the
first thickness adjusting layer and the second thickness adjusting
layer have different thicknesses. With this arrangement, different
oscillation frequencies can be output by means of a single
oscillation device.
[0038] While the thickness of the first and second thickness
adjusting layers is not subjected to any particular limitations, it
may typically be selected from the range of not less than 5 nm and
not more than 100 nm. However, the first and second thickness
adjusting layers preferably have different thicknesses selected
from the range of not less than 5 nm and not more than 60 nm in
order to produce a large change in frequency by switching the bias
voltage. The difference of thickness between the first thickness
adjusting layer and the second thickness adjusting layer is not
less than 10 nm, preferably not less than 20 nm, more preferably
not less than 40 nm from the viewpoint of securing a large
difference of oscillation frequency.
[0039] The resonant tunneling diode is provided with a first
electrode and a second electrode for the purpose of applying a bias
voltage thereto. The switching unit is arranged so as to operate as
a unit for changing the oscillation frequency by inverting the
polarity of the bias voltage being applied to the resonant
tunneling diode for the first electrode and the second
electrode.
[0040] The gain medium preferably has a plurality of quantum well
layers and a barrier layer is arranged between the quantum well
layers.
[0041] Preferably, the first thickness adjusting layer is arranged
between the first barrier layer and the first contact layer and
formed as non-doped layer. Similarly, the second thickness
adjusting layer is preferably arranged between the second barrier
layer and the second contact layer and formed as non-doped
layer.
[0042] The quantum well layer is formed from a material composed of
indium gallium arsenide (InGaAs). The first and second barrier
layers are formed from a material composed of aluminum arsenide or
indium aluminum arsenide (AlAs, InAlAs). The first and second
thickness adjusting layers are typically formed from a material
composed of non-doped indium gallium arsenide (InGaAs). The
composition ratio of the material of each of the above-listed
layers may be appropriately selected.
[0043] The switch for switching the polarity of the bias voltage is
not subjected to any particular limitations so long as it can
invert the polarity of the bias voltage being applied to the
resonant tunneling diode. When the polarity of the bias voltage is
inverted, the absolute value of the bias voltage before the
switching is not necessarily required to be equal to the absolute
value of the bias voltage after the switching. For instance, the
bias voltage may be gradually shifted from a certain positive
voltage value to a certain negative voltage value.
[0044] The oscillation device may be provided with an antenna
resonator. For example, it may be provided with a planar antenna
type resonator for confining an electromagnetic wave in the gain
medium having a gain of the electromagnetic wave. The gain medium
is formed by one or more quantum well layers and a plurality of
barrier layers separating the quantum well layers and the
transition of carriers among sub-bands of the one or more quantum
well layers is based on the resonant tunneling diode where a gain
is produced by way of photon assisted tunneling. A unit for
injecting carriers into the gain medium of the resonant tunneling
diode is provided. The unit for injecting carriers includes first
and second contact layers made of a semiconductor illustrating
conductive properties and sandwiched between the first and second
thickness adjusting layers and the barrier layers. The first
thickness adjusting layer and the second thickness adjusting layer
are made to illustrate different thicknesses.
[0045] The oscillation frequency of such an oscillation device can
be adjusted to the characteristic vibration spectrum of the
substance to be detected. Thus, an inspection apparatus for
inspecting the presence or absence of the substance to be detected
can be formed by using such an oscillation device.
[0046] An inspection apparatus may be formed in the following
manner. It is formed by using a plurality of oscillators according
to the present invention, each having different oscillation
frequencies, a plurality of optical systems such as mirrors and
lenses for irradiating the electromagnetic wave outputs thereof
onto the object of inspection and detectors arranged corresponding
to the frequencies of the respective oscillators so as to detect
the electromagnetic waves from the object of inspection
independently and respectively.
[0047] An oscillation device according to the present invention can
be used for having communications by means of a frequency-shift
keying system, where the frequency of the oscillation device is
switched by way of a unit for switching the polarity of the bias
voltage being applied to a resonant tunneling diode. Thus, a novel
communication system can be formed by employing an oscillation
device according to the present invention as light source.
[0048] Now, an oscillation device according to the present
invention will be described in greater detail by referring to FIGS.
1A and 1B. While the oscillation device is provided with a slot
antenna structure in FIGS. 1A and 1B, such an antenna may be
provided only when necessary and the antenna is not limited to a
slot antenna structure.
[0049] FIGS. 1A and 1B are a schematic perspective view of a THz
oscillation device according to the present invention and a
schematic cross sectional view of the semiconductor part thereof,
illustrating the structure thereof. Ti/Pd/Au layers 2 and 3 that
operate as electrodes and antennas are formed on a substrate 1 with
an insulating layer 6 interposed between them. A window region 4 is
arranged at the upper electrode 3 by partly removing the electrodes
2 and 3 to produce a slot antenna structure. According to the
present invention, a resonator is formed by the slot antenna and
the length of the window region as indicated by arrow heads is the
factor that determines the oscillation frequency. Reference symbol
5 denotes a semiconductor region formed to illustrate a post-shaped
profile. FIG. 1B illustrates a cross sectional view of the
semiconductor region taken along line 1B-1B in FIG. 1A.
[0050] The following layers are formed on the semi-insulating InP
substrate 1: a first n.sup.+-InGaAs contact layer 15, an n-InGaAs
layer 14, a first non-doped InGaAs thickness adjusting layer 13, a
first non-doped barrier layer InAlAs layer 12, a non-doped InGaAs
quantum well layer 11, a second non-doped InAlAs barrier layer 10,
a second non-doped InGaAs thickness adjusting layer 9, an n-InGaAs
layer 8 and a second n.sup.+-InGaAs layer 7. These layers are
typically formed by crystal growth by means of a molecular beam
epitaxy apparatus and then by dry etching such as an ICP process so
as to make them illustrate a rectangular post-shaped profile with a
width of 2.5 .mu.m.
[0051] A voltage is applied to these layers by way of the
electrodes 2 and 3 that are connected to a power source 21 or 22.
An active layer having a double-barrier quantum well structure is
formed by the layers 10, 11 and 12 to operate as a resonant
tunneling diode having a negative resistance and produce a gain
section.
[0052] The thickness adjusting layer collectively refers to the
non-doped layer part that is sandwiched between the doped layer and
the outermost barrier layer that participates in the active layer
and conventionally referred to as spacer layer. An ordinary spacer
layer is arranged to a thickness of 5 nm in order to prevent a
dopant from diffusing into the active layer and degrading the
quality of the active layer when the crystal is made to grow by
epitaxial growth. This is because the traveling speed of carriers
falls and the resistance rises when the thickness is too large.
[0053] According to the present invention, the thickness adjusting
layer refers to an ordinary spacer layer where an adjusting layer
is arranged to control a depletion layer. The parasitic capacitance
of the device changes because the thickness of the depletion layer
changes as a function of the thickness of the collector layer as
thickness adjusting layer that takes in carriers, or the side
connected to the anode when carriers are electrons.
[0054] FIG. 2 is a graph illustrating the relationship between the
thickness of the collector layer and the oscillation frequency of
an oscillation device according to the present invention. In FIG.
2, the assumed device parameters of the device include an antenna
length of 50 .mu.m and a peak current density Jp=200 kA/cm.sup.2
for the V-I characteristic as illustrated in FIG. 12. It will be
seen that the oscillation frequency remarkably changes from 300 GHz
to 550 GHz as the thickness of the collector layer is made to vary
from 5 nm to 60 nm. When the thickness of the collector layer is
made to exceed 60 nm, it no longer provides a region where carriers
can be conducted ballistically so that the conduction velocity
falls and hence the oscillation frequency can not be made to vary
any further.
[0055] As seen from FIG. 2, if the thickness of the thickness
adjusting layer at the emitter side where carriers are injected is
assumed to be substantially invariable, the oscillation device
oscillates with oscillation frequencies of 480 GHz and 370 GHz when
the thickness of the thickness adjusting layer at the collector
layer side is made to be equal to 40 nm and 20 nm respectively.
Thus, if the thickness of the first thickness adjusting layer and
that of the second thickness adjusting layer are made to be equal
to 40 nm and 20 nm respectively, the thickness adjusting layer that
operates as collector layer is inverted according to the polarity
of the voltage being applied to the device. The net result is that
a device having a collector layer whose thickness varies is
provided and driven to operate. This can be realized by operating
the switch 20 for connecting the device to one of the power sources
having opposite polarities to another. Alternatively, a power
supply unit that can output either with the positive polarity or
the negative polarity may be provided to omit the switch.
[0056] FIGS. 3A and 3B schematically illustrate how the
characteristics of an oscillation device changes as a function of
the thickness of the collector layer. In FIGS. 3A and 3B, only a
conduction band is illustrated on an assumption that carriers are
electrons. FIG. 3A illustrates an instance where the first
thickness adjusting layer operates as a collector layer with d2=40
nm. In this case, the electrode 3 is connected to the cathode while
the electrode 2 is connected to the anode and the device oscillates
at 480 GHz because the arrangement corresponds to point (i) in FIG.
2. On the other hand, FIG. 3B illustrates an instance where the
polarity is inverted and the second thickness adjusting layer
operates as a thin collector layer with d3=20 nm. Thus, the device
oscillates at 370 GHz because the arrangement corresponds to point
(ii) in FIG. 2. In this case, carriers diffuse sufficiently at the
emitter side and the difference of thickness of the thickness
adjusting layer does not have any influence. In other words, d1 and
d4 do not illustrate a large difference and, if they do, they do
not affect the operation speed. Thus, the oscillation frequency is
determined substantially by the thickness of the thickness
adjusting layer that operates as a collector layer.
[0057] This characteristic can be understood by seeing the
equivalent circuit of the resonant tunneling diode illustrated in
FIG. 4. In FIG. 4, Ccont and Cact respectively denote the capacity
component of the contact layer and that of the active layer and
Lact denotes the inductance component of the active layer, whereas
Rcont, Rpost and Ract respectively denote the resistance component
of the contact layer, that of the post section and that of the
active layer. Cact=.epsilon.S/d in FIGS. 3A and 3B indicates that
Cact of the equivalent circuit in FIG. 4 changes to reflect the
difference of the thicknesses d2 and d3. As a result, the entire
resonance frequency of the same slot antenna type resonator changes
and appears as a switch of the oscillation frequency. This can be
understood because the Q value of a planar antenna resonator is
generally relatively small and a change in the parasitic
capacitance of the device is apt to be reflected to a switch of the
oscillation frequency.
[0058] From the above, a device that oscillates with different
frequencies simply can be provided by making the thickness of the
first thickness adjusting layer and that of the second thickness
adjusting layer asymmetric and inverting the polarity of the bias
voltage. While carriers are electrons in the above description, a
two-frequency oscillation device that operates in the valence
electron band on the same operation principle can be provided by
using holes.
[0059] Now, the present invention will be described further by way
of examples.
EXAMPLE 1
[0060] The first example of the present invention is an oscillation
device having a structure as illustrated in FIGS. 1A and 1B with an
antenna length of 50 .mu.m and a semiconductor layer 5 forming a
resonant tunneling diode and illustrating a post-shaped profile
with a width of 2.5 .mu.m. The semiconductor layers forming the
device have the respective thicknesses as listed below: [0061]
n.sup.+-InGaAs contact layer 15: 400 nm; [0062] n-InGaAs layer 14:
50 nm; [0063] first non-doped InGaAs thickness adjusting layer 13:
40 nm; [0064] first non-doped AlAs barrier layer (In=0) 12: 1.5 nm;
[0065] non-doped InGaAs quantum well layer 11: 4.5 nm; [0066]
second non-doped AlAs barrier layer (In=0) 10: 1.5 nm; [0067]
second non-doped InGaAs thickness adjusting layer 9: 30 nm; [0068]
n-InGaAs layer 8: 50 nm; and [0069] a second n.sup.+-InGaAs contact
layer 7: 20 nm.
[0070] FIG. 5 is a graph illustrating the V-I characteristic of the
device observed when the device is electrically energized. In the
case of FIG. 3A where the thickness adjusting layer having a large
thickness is operated as collector layer, a maximum gain is
obtained with V2 at the negative bias side because a higher
electric field is required. Conversely, in the case of FIG. 3B, a
maximum gain is obtained at the positive bias side. In both cases,
the device oscillates near the bias voltage. More specifically, the
device oscillates at 370 GHz with V1=0.4 V and at 480 GHz with
V2=-0.5 V.
[0071] As V1 and V2 are made to vary in a region where a negative
resistance is generated and hence a gain exists as indicated by
arrow heads in FIG. 5, the oscillation wavelength can be made to
change slightly (by about several %) because the thickness of the
depletion layer in the same collector layer changes slightly.
Therefore, the oscillation frequency of 370 GHz can be made
variable by about .+-.10 GHz while the oscillation device is driven
to oscillate at that frequency. Similarly, the oscillation
frequency of 480 GHz can be made variable by about .+-.10 GHz while
the oscillation device is driven to oscillate at that
frequency.
[0072] To actually switch the oscillation frequency, V1 or V2 may
be selected by the switch 20 arranged at the power sources 21 and
22 as illustrated in FIG. 1A. The bias voltage may be lowered
gradually to 0 and then raised again after switching the polarity
to drive the oscillation device in order to protect the device.
Alternatively, the two frequencies may be switched quickly for FSK
(frequency-shift keying) modulation so as to drive the device to
operate as light source for communications in a sub-THz band.
EXAMPLE 2
[0073] In the second example of the present invention, the
semiconductor post 60 that forms a resonant tunneling diode is
arranged not at the center of the window region 61 for forming a
slot antenna but at a position slightly shifted from the center as
illustrated in FIG. 6.
[0074] With this arrangement, the oscillation frequencies can be
raised further. When a structure same as that of Example 1 is
employed and the semiconductor post 60 is arranged at a position
displaced by 15 .mu.m from the central position, each of the
oscillation frequencies can be raised by about 20% and the width of
the two frequency oscillation can be adjusted so as to make it
broader.
EXAMPLE 3
[0075] In the third example of the present invention, the
oscillation frequencies are adjusted by changing the longitudinal
length of the resonator, or the slot antenna in FIG. 1A.
[0076] FIG. 7 is a graph illustrating computation examples for
designing the oscillation frequencies of the oscillation device of
this example. It illustrates the oscillation frequencies
corresponding to the thickness of the thickness adjusting layer
that operates as collector when the antenna length is reduced to 30
.mu.m, 15 .mu.m and 10 .mu.m. Note that the semiconductor layer 5
is a post with a width of 1.8 .mu.m and the peak current density is
400 kA/cm.sup.2.
[0077] Then, if the antenna length is 30 .mu.m and the thicknesses
of the thickness adjusting layers are 40 nm and 20 nm, it will be
seen that two oscillation frequencies of 610 GHz and 770 GHz are
available. Similarly, a desired two-frequency oscillation device
can be provided by selecting desired oscillation frequencies from
FIG. 7.
[0078] An oscillation device according to the present invention can
be made to oscillate at frequencies higher than those of Examples 1
and 2 by reducing the width of the post and the antenna length.
EXAMPLE 4
[0079] In the fourth example of the present invention, the antenna
length of the slot antenna of FIG. 1A is made equal to 30 .mu.m
while the semiconductor layer 5 is formed as a 2.3 .mu.m wide post
and a resonant tunneling diode having a triple barrier quantum well
is employed. The semiconductor layers forming the device have the
respective thicknesses as listed below: [0080] non-doped InGaAs
thickness adjusting layer 90: 40 nm; [0081] non-doped AlAs barrier
layer 91: 1.3 nm; [0082] non-doped InGaAs quantum well layer 92:
7.6 nm; [0083] non-doped InAlAs barrier layer 93: 2.6 nm; [0084]
non-doped InGaAs quantum well layer 94: 5.6 nm; [0085] non-doped
AlAs barrier layer 95: 1.3 nm; and [0086] non-doped InGaAs
thickness adjusting layer 96: 20 nm.
[0087] Note that the above-listed thicknesses are only examples and
the present invention is by no means limited thereto.
[0088] FIG. 8 is a graph illustrating design examples for designing
the oscillation frequencies of the oscillation device of the
Example 4. In FIG. 8, B denotes the oscillation frequency
dependency of the thickness adjusting layer at the collector side
when a 5.6 nm quantum well layer 94 is at the emitter side and A
denotes the oscillation frequency dependency of the thickness
adjusting layer at the collector side when a 7.6 nm quantum well
layer 92 is at the emitter side. From the graph, the oscillation
frequency can be made equal to 590 GHz when the 40 nm thickness
adjusting layer is at the collector side, and equal to 210 GHz when
the 20 nm thick adjusting layer is at the collector side. The peak
current densities for them are respectively 90 kA/cm.sup.2 and 280
kA/cm.sup.2. In this way, an oscillation device of the triple
barrier layer type can raise the frequency difference when compared
with a device of the double barrier layer type.
[0089] While the above-described devices employ a slot antenna as
antenna resonator, antenna of some other structure such as a patch
antenna or dipole antenna may alternatively be employed.
[0090] Additionally, oscillation devices are described in the above
examples, a similar arrangement can be used to detect an
electromagnetic wave incoming from outside with a high sensitivity
by adjusting the bias voltage to the state immediately before
oscillation in advance. With such an arrangement, the incoming
electromagnetic wave can be detected with a high sensitivity only
when it has a frequency close to the oscillation frequency of the
device. In other words, such an arrangement can be operated as a
detector equipped with a band frequency filter. Additionally, two
different frequencies can be detected by switching the polarity of
the bias voltage.
EXAMPLE 5
[0091] The fifth example provides an inspection apparatus for
inspecting an object that employs an oscillation device according
to the present invention.
[0092] As illustrated in FIG. 10, a plurality of oscillation
frequencies f1 through f8 are produced by means of oscillators 70a
through 70d according to the present invention, each of which
oscillates in two frequencies. Each of the electromagnetic waves is
made to propagate as a collimated beam by means of a parabolic
mirror 74. Beams of light irradiated onto and transmitted through
the object 72 of inspection are collected and received by
respective detectors 71a through 71d. While the detectors are
arranged to receive transmitted beams of light in this example,
detectors may alternatively be so arranged as to receive reflected
beams of light.
[0093] If the substance to be detected illustrates one or more
specific absorption spectrums for the frequencies f1 through f8, if
the object 72 of inspection contains the substance or not can be
determined by making the inspection apparatus store patterns of
combination of different intensities in advance.
[0094] FIG. 11 is a graph schematically illustrating an exemplary
fingerprint spectrum of a substance to be detected. The substance
illustrates absorption peaks at frequencies f1, f6 and f7.
Therefore, if the absorption pattern of the substance to be
detected is stored in the inspection apparatus in advance and
collated with the information that the detected absorption pattern
illustrates weak output levels at f1, f6 and f7 but strong output
levels at other frequencies, the substance to be detected is
contained in the object of inspection.
[0095] Such an inspection apparatus can find applications in the
field of detection of hazardous and prohibited substances in
airports, in the field of inspection of items being distributed by
post or as freights and in the field of inspection of industrial
products at manufacturing plants.
[0096] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
present invention is not limited to the disclosed exemplary
embodiments. The scope of the following claims is to be accorded
the broadest interpretation so as to encompass all such
modifications and equivalent structures and functions.
[0097] This application claims the benefit of Japanese Patent
Application No. 2007-213644, filed on Aug. 20, 2007, which is
hereby incorporated by reference herein in its entirety.
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